It’s Always Sunny in Philadelphia is my current favorite TV show. It could never usurp Arrested Development as my favorite show ever, but it ain’t far behind. The show is essentially about a group of five Philadelphians (including Danny DeVito), who are incredibly self-centered and just horrible people in general, having various adventures. Hilarity ensues. If you’re easily offended you probably won’t like the show. If you have a sense of humor, and actually understand that when the characters are being insensitive to various groups they’re not supporting insensitivity (in fact, they’re doing quite the opposite), then you’ll probably love the show.
As for the shirt, I think it’s pretty self-explanatory. It’s Always Sunny in Philadelphia.